With present-day audio apparatus, there is the possibility of switching the same to a mute condition (muting) during specific switching operations to avoid the switching noise becoming audible in the loudspeakers.
An example in this respect are RDS radio apparatus capable of processing RDS signals. RDS stands for Radio Data System and means that the broadcasting station broadcasts signals in addition to the regular audio signals. These inaudible signals serve, for example, to display the name or an abbreviation of the radio station received at the particular time and contain information on additional transmitter frequencies of the received radio program other than the frequency received at a particular time.
There are various suggestions to protect the listener of an RDS receiver from noise due to a too poor receiving quality or from other disturbing noise.
The document DE 42 27 680 A1 discloses an FM receiver with RDS evaluation which, during receipt of an RDS program on a first frequency, switches temporarily to at least one second alternative frequency of the received program in order to examine the receiving quality thereof, with the receiver being muted on the reproduction side during the examination.
The document DE 43 18 667 A1 reveals an audio apparatus having an RDS receiver, a CD player and a tape player, wherein, when the receiving quality threatens to become too poor, for example in case of an RDS car radio upon entering a tunnel, unpleasant noise caused thereby is avoided in that the apparatus. During the time in which the receiving quality drops below a specific threshold value, the apparatus automatically switches over to reproduction of the audio signal from the CD player or the tape player, respectively.
The document DE 43 09 518 A1 discloses an RDS receiver in which the LF signal is attenuated temporarily in case of too poor receiving quality. To this end, an auxiliary signal dependent on the field strength and an additional auxiliary signal are continuously derived from the demodulated signal, and these two auxiliary signals are combined to form the quality signal. The one auxiliary signal is used to determine whether there are amplitudes of signal portions above 60 kHz which are above a threshold value. The second auxiliary signal is used for assessing whether or not the two sidebands of the two-sideband amplitude modulation are arranged symmetrically with respect to the carrier. An asymmetry in this respect indicates that an audible disturbance is present in the LF signal.
A feature of RDS radio apparatus consists in that, when the receiving quality of the radio program received at the particular moment drops below a predetermined quality value, automatic switching over to another one of the receiving frequencies of the same program, which are communicated in the RDS signal, takes place.
Solutions have been found permitting the switching operation to be performed so rapidly that the listener does not at all notice switching to another receiving frequency. During such a frequency switching jump, the audio apparatus is briefly switched to a mute condition. Abrupt switching over to the mute condition causes cracking noise at the loudspeaker output. This can be avoided by a soft muting transition or soft muting, and the switching operation thus can be rendered even more inconspicuous for the listener.
With a conventional muting circuit for soft muting of an audio signal, a capacitor is used that is charged or discharged at a predetermined time constant, depending on whether a soft transition into the mute state (muting) or a soft transition from the mute state (demuting) takes place. The increasing charging voltage or the decreasing discharging voltage of the capacitor is utilized as an attenuation ramp by means of which the amplitude of the signal to be attenuated is increasingly reduced down to zero during muting and, upon leaving the mute condition, the amplitude of the signal is increasingly raised starting from zero. However, in reasonably priced circuits this is achieved only incompletely. Almost only the envelope curve of the signal to be muted is increasingly decreased or raised in accordance with the attenuation ramp, since in case of simple circuits only, a clamping or limiting operation takes place in which the signal to be muted is limited in increasingly stronger or lesser manner in accordance with the attenuation ramp.
What is problematic with such soft muting of conventional nature is that the limitation, resulting in trimming of the amplitude, causes a harmonic distortion of the limited audiG signal and necessitates a capacitor for such soft muting. Audio signal processors performing such signal processing operations nowadays are designed as monolithic high-integrated circuits. Capacitors, as required for soft muting, cannot be accommodated with reasonable expenditure on such integrated circuits due to the long time constants required, and thus must be designed as external capacitors. By determining the capacitor size, the user can determine only a time constant for the muting operation. Such an external capacitor does not only result in an increase of the overall costs, but also necessitates an additional terminal pin of the integrated circuit. Integrated circuits having a large number of terminal pins necessitate complicated chip housings, entailing corresponding costs.